This invention relates to a reading method for reading radiation image information stored on a stimulable phosphor sheet and to a radiation image reading apparatus.
Many radiation image reading apparatuses are used in which image information is outputted as digital data in order to store and transmit radiation image information generated at hospitals. A radiation image reading apparatus in which a stimulable phosphor sheet is used, is a well known example of the radiation image reading apparatus which outputs this type of digital data.
The stimulable phosphor sheet is one in which a thin layer of a stimulable phosphor is formed on a support material and covered with a protective layer. The stimulable phosphor accumulates radiation energy such as the irradiated X-rays and the like, and also releases the accumulated energy as photo-stimulated luminescence by being irradiated with excitation light of a prescribed wavelength.
When the stimulable phosphor sheet is used in X-ray radiography, the radiation energy which has passed through an object can be accumulated in the stimulable phosphor sheet. Photo-stimulated luminescence can be taken out by subsequently irradiating the stimulable phosphor sheet with excitation light and then the radiation energy is taken out as an electric signal by a photoelectric conversion element such as a photomultiplier.
As described in Patent Document 1, the stimulable phosphor sheet may be the coated type in which a stimulable phosphor such as BaFX:Eu (X=Br or I) and the like, is dispersed in a suitably selected solvent and the dispersant is coated on a support material and then dried and hardened and a stimulable phosphor layer is thereby formed.
In addition, as described in Patent Document 2 for example, there is a vapor deposit type in which a vapor phase deposition method such as vapor deposition or the like is used and a stimulable phosphor such as CsBr:Eu is crystallized on a support material and a stimulable phosphor layer is thereby formed. In the stimulable phosphor sheet of the vapor deposit type, columnar crystal of the stimulable phosphor is formed on a support material.
In the vapor deposit type stimulable phosphor sheet, because dispersion of the excitation light and the photo-stimulated luminescence in the horizontal direction can be controlled, the sharpness of the image can be remarkably improved. The inventors of the present invention realized however, that on the other hand, if the stimulable phosphor layer is made thick (a range from several tens of micrometers to a few millimeters) in order to increase luminance and sensitivity, the columnar crystal will become fragile and tend to be damaged by external force and the like. In particular, it becomes likely for the columnar crystal to receive damage from external forces acting in the perpendicular direction to the growth direction. Thus attention must be paid such that no impact is received during transportation, radiographing or image reading.
An example of the radiation image reading apparatus which reads radiation image information from the stimulable phosphor sheet is that described in Patent Document 3. In this apparatus, a flexible coating-type stimulable phosphor sheet is taken out from a cassette using a suction cup or the like and a contact conveyance method is used in which the stimulable phosphor sheet is nipped between paired rollers and conveyed. In this contact conveyance method, because the surface of the stimulable phosphor is pressed by the roller, if dirt or dust is attached to the roller, the phosphor surface becomes stained and scratched and this causes a critical drawback in that the lifespan of the stimulable phosphor sheet is shortened. In addition, because the scratches or stains on the surface of the stimulable phosphor appear as noise on the image that has been read, it becomes impossible to provide a high quality image for diagnosis. Furthermore, because external forces act on the stimulable phosphor sheet, the vapor deposition type stimulable phosphor sheet is unsuitable.
In order to eliminate the shortcomings of the contact conveyance method, a non-contact conveyance method is proposed in which the stimulable phosphor sheet is attached to a rigid plate member, and only the rigid plate member is contacted (the stimulable phosphor surface is not contacted). The radiation image reading apparatus using this method is described in Patent Document 4 for example.
The stimulable phosphor sheet which this apparatus reads is installed and used in a thin box-shaped portable casing called a cassette. The cassette is composed of a back base to which the stimulable phosphor sheet is attached and a front base which covers the stimulable phosphor sheet.
The back base has a locking pawl which engages with the front base. The locking pawl is configured so as to switch between an engaged state (lock ON) and a disengaged state (lock OFF) with respect to the front base, using a push-latch mechanism. In the engaged state the back base and the front base are joined, while in the disengaged state the back base and the front base can be separated from each other. The back base is formed from a material having a ferromagnetic substance such that it can be stuck to a moving plate described hereinafter using magnetic force.
There are a conveyance device, a sub-scanning device and a main scanning device inside the radiation image reading apparatus described in Patent Document 4. The cassette that is inserted into the radiation image reading apparatus through the reading slot is first caught by the front base using the conveyance device and disengaged using the lock ON/OFF mechanism provided at the conveyance device. Next the back base is delivered to the moving plate of the sub-scanning device due to the rotation of the conveyance device.
The sub-scanning device is composed of a moving plate and a sub-scanning rail which moves the moving plate vertically. The moving plate has a magnet, and when the back base contacts the moving plate due to the rotation of the conveyance device, the back base is stuck by the magnet. Next the back base and the front base are separated when the conveyance device rotates in the direction away from the moving plate.
The moving plate to which the back base is stuck is conveyed upward along the sub-scanning rail (sub-scanned). During the sub-scanning operation, the stimulable phosphor sheet is scanned with a laser beam having a prescribed wavelength using the main scanning device and then photo-stimulated luminescence is taken out as electrical signals using a photoelectric conversion element such as photomultiplier.
However, in the radiation image reading apparatus described in Patent Document 3, because the radiation image information is read while the stimulable phosphor sheet is pulled downwards from the cassette that is fixed on the upper portion of the apparatus, the position of the insertion opening of the cassette is high and it becomes necessary for the radiographic technician (radiologist) or the like to lift the cassette which has been carried by being held in his hand or between his arm and side after radiographing, against gravity up to the high position of the insertion opening, and this was a large burden on the technician.
In the radiation image reading apparatus of Patent Document 4, because the rotation mechanism for rotating the conveyance device at the lower portion of the apparatus, the position of the cassette reading slot is high. In addition, because the reading slot has a support member for supporting the cassette that has been slantingly inserted, the user has a trouble of lifting the cassette higher than the support member. Furthermore, because the cassette is separated by the rotation movement, the area for installing the apparatus (foot print) and the operation space required around the apparatus, especially front space of the cassette reading slot, become large.
[Patent Document 1] Japanese Patent Application Laid-Open 2003-255096 Publication
[Patent Document 2] Japanese Patent Application Laid-Open 2004-239713 Publication
[Patent Document 3] Japanese Patent Application Laid-Open 2004-109252 Publication
[Patent Document 4] Japanese Patent Application Laid-Open 2002-156716 Publication